The present invention relates generally to the fabrication of pasted electrodes for electrochemical devices such as batteries and fuel cells. More specifically, it relates to an automated apparatus for rapidly and uniformly pasting a solid particulate slurry into a continuous web of a substrate material.
In rechargeable electrochemical cells, weight and portability are important considerations. It is also advantageous for rechargeable cells to have long operating lives without the necessity of periodic maintenance. Rechargeable electrochemical cells are used in numerous consumer devices such as calculators, portable radios, and cellular phones. They are often configured into a sealed power pack that is designed as an integral part of a specific device. Rechargeable electrochemical cells can also be configured as larger xe2x80x9ccell packsxe2x80x9d or xe2x80x9cbattery packsxe2x80x9d.
Rechargeable electrochemical cells may be classified as xe2x80x9cnonaqueousxe2x80x9d cells or xe2x80x9caqueousxe2x80x9d cells. An example of a nonaqueous electrochemical cell is a lithium-ion cell which uses intercalation compounds for both anode and cathode, and a liquid organic or polymer electrolyte. Aqueous electrochemical cells may be classified as either xe2x80x9cacidicxe2x80x9d or xe2x80x9calkalinexe2x80x9d. An example of an acidic electrochemical cell is a lead-acid cell which uses lead dioxide as the active material of the positive electrode and metallic lead, in a high-surface area porous structure, as the negative active material. Examples of alkaline electrochemical cells are nickel cadmium cells (Nixe2x80x94Cd) and nickel-metal hydride cells (Ni-MH). Ni-MH cells use negative electrodes having a hydrogen absorbing alloy as the active material. The hydrogen absorbing alloy is capable of the reversible electrochemical storage of hydrogen. Ni-MH cells typically use a positive electrode having nickel hydroxide as the active material. The negative and positive electrodes are spaced apart in an alkaline electrolyte such as potassium hydroxide.
Upon application of an electrical potential across a Ni-MH cell, the hydrogen absorbing alloy active material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical discharge of a hydroxyl ion, forming a metal hydride. This is shown in equation (1): 
The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released from the metal hydride to form a water molecule and release an electron.
Hydrogen absorbing alloys called xe2x80x9cOvonicxe2x80x9d alloys result from tailoring the local chemical order and local structural order by the incorporation of selected modifier elements into a host matrix. Disordered hydrogen absorbing alloys have a substantially increased density of catalytically active sites and storage sites compared to single or multi-phase crystalline materials. These additional sites are responsible for improved efficiency of electrochemical charging/discharging and an increase in electrical energy storage capacity. The nature and number of storage sites can even be designed independently of the catalytically active sites. More specifically, these alloys are tailored to allow bulk storage of the dissociated hydrogen atoms at bonding strengths within the range of reversibility suitable for use in secondary battery applications.
Some extremely efficient electrochemical hydrogen storage alloys were formulated, based on the disordered materials described above. These are the Tixe2x80x94Vxe2x80x94Zrxe2x80x94Ni type active materials such as disclosed in U.S. Pat. No. 4,551,400 (xe2x80x9cthe ""400 patentxe2x80x9d) the disclosure of which is incorporated herein by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the ""400 patent utilize a generic Tixe2x80x94Vxe2x80x94Ni composition, where at least Ti, V, and Ni are present and may be modified with Cr, Zr, and Al. The materials of the ""400 patent are multiphase materials, which may contain, but are not limited to, one or more phases with C14 and C15 type crystal structures.
Other Tixe2x80x94Vxe2x80x94Zrxe2x80x94Ni alloys, also used for rechargeable hydrogen storage negative electrodes, are described in U.S. Pat. No. 4,728,586 (xe2x80x9cthe ""586 patentxe2x80x9d), the contents of which is incorporated herein by reference. The ""586 patent describes a specific sub-class of Tixe2x80x94Vxe2x80x94Nixe2x80x94Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The ""586 patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them. Other hydrogen absorbing alloy materials are discussed in U.S. Pat. Nos. 5,096,667, 5,135,589, 5,277,999, 5,238,756, 5,407,761, and 5,536,591, the contents of which are incorporated herein by reference.
The hydrogen storage alloy negative electrode may be paste type or non-paste type. Non-paste type electrodes are formed by pressing or compacting the active hydrogen absorbing alloy onto a conductive substrate. A method of fabricating non-paste types negative electrodes is disclosed in U.S. Pat. No. 4,820,481 (xe2x80x9cthe ""481 patentxe2x80x9d) the disclosure of which is incorporated herein by reference. The present invention is directed to an apparatus for making paste type electrodes and, in particular, paste type hydrogen absorbing alloy electrodes.
Disclosed herein is an apparatus for pasting a slurry onto a substrate, comprising: a paster for loading the slurry onto the substrate to form a loaded substrate; a pump for delivering the slurry from a storage container to the paster; and a thickness sensor for measuring the thickness of the loaded substrate and forming a control signal corresponding to the thickness, the speed of the pump being responsive to the control signal.
Also disclosed herein is a paster for loading a slurry onto a substrate to form a loaded substrate, comprising: a cavity defined within the paster, a substrate inlet allowing the substrate to enter the cavity, at least one slurry inlet allowing the slurry to enter the cavity, the paster being adapted so that the slurry substantially fills the cavity, the substantially filled cavity creating a pressure forcing the slurry onto the substrate to form a loaded substrate.